Give it a good wire-brushing before use to clean off crud and any loose rust will fall off.
Some of those cheap "cast iron" grill grates show rust after some use and people just brush them off. What you want to do can't come close to looking as bad.
Welded (well, scabbed) lots of 409 "stainless" car exhausts back on after they fell off to squeak a bit more life out of a junker. Used O/A and RG45 filler. Not a first-rate approach but it holds.....for awhile.
Harris dual-stage O/A
Lincoln AC/DC buzzbox
Hobart IronMan 210
Lincoln PowerMig 135
Miller 3035 spoolgun
Thermal Arc 185
Thermadyne Cutmaster 52
Angle grinders, vicegrips, the usual suspects
Two hands, tired body, not enough time...